1. Field of the Invention
The subject invention relates generally to catheters and, more particularly, to a thermodilution catheter assembly for cardiac monitoring and method.
2. Background Art
Catheters are commonly used in cardiac monitoring. One such catheter, sometimes referred to as a flow-directed thermodilution catheter, is used as a diagnostic tool for obtaining hemodynamic pressures of the heart. In addition, such catheters are frequently used for measuring cardiac output. The catheter is provided with three lumens, including a first lumen for inflating a balloon located at the distal catheter tip. A second lumen is included which is open at the distal catheter tip and which is used for measuring pressure. The third lumen extends along the length of the catheter and terminates typically 30 cm from the distal catheter tip. The catheter further includes a thermistor which is positioned between the distal tip of the catheter and the third lumen opening.
In operation, the catheter is introduced into a central vein and advanced toward the right heart through the superior vena cava. When the distal tip of the catheter is positioned within the vena cava, a small balloon located at the tip is inflated through the inflation lumen. The balloon directs the catheter along the flow of blood as the catheter is further advanced from the vena cava through the right atrium, right ventricle and into the main pulmonary artery. The catheter is advanced until the balloon impinges upon the walls of an individual pulmonary artery or capillary. In this position, referred to as the wedged position, the inflated catheter balloon seals off the inflow of blood through the artery or capillary. The hemodynamic pressure may then be measured using a pressure measuring gauge coupled to the proximal port of the second lumen. In addition, a bolus of cold water may be injected through the third lumen into the heart and out of the opening located 30 cm from the distal tip of the catheter. The temperature change caused by thermodilution is then measured downstream at the thermistor and used to calculate the cardiac output in accordance with the well-known Fick principle.
A principle disadvantage of the cardiac monitoring catheters now in use is that it is frequently difficult to reliably obtain a wedge pressure measurement after the catheter has been placed and sewn into position. Once a pressure measurement has been made, the catheter balloon is deflated. The balloon is reinflated for subsequent pressure measurements. Success in obtaining such pressure measurements is dependent upon the distal tip of the catheter remaining in the position of original placement. Migration of the catheter tip out of the original pulmonary artery and into a different pulmonary artery may prevent the balloon from completely occluding the artery. Accordingly, it would not be possible to obtain subsequent pressure measurements. Conversely, the catheter tip may migrate into a permanently wedged position wherein the artery remains occluded even when the balloon has deflated. This condition is likely to cause a pulmonary infarction due to prolonged interruption of pulmonary blood flow and may also cause erosion injury to the pulmonary artery wall.
The present invention overcomes the above-noted disadvantages of catheters presently in use. Migration of the distal tip of the catheter is minimized so that it is generally always possible to obtain a pressure measurement. In addition, the disclosed catheter will never become permanently wedged. These and other advantages of the subject invention will become apparent to a person having ordinary skill in the art upon a reading of the following Best Mode for Carrying out the Invention together with the drawings.